The present invention relates to refrigeration systems and, more particularly, relates to methods and control systems for protecting a refrigeration system compressor against flooding at startup of the refrigeration system.
Conventional refrigeration systems utilize a recirculating refrigerant for removing heat from a low temperature side of the refrigeration system and for discharging heat at a high temperature side of the refrigeration system. The work input necessary to operate the refrigeration system is provided by a motor driven compressor which receives low pressure gaseous refrigerant and compresses it to a high pressure. This high pressure gaseous refrigerant is supplied to a condenser where heat is removed from the gaseous refrigerant to condense it to a liquid. This liquid refrigerant is then supplied through an expansion valve to an evaporator wherein heat is transferred from a heat transfer fluid to the liquid refrigerant to evaporate the liquid refrigerant. The heat transfer fluid is thereby cooled and then used to cool a load, such as to cool a building. The evaporated refrigerant from the evaporator is returned to the compressor for recirculation through the refrigeration system.
During off periods of the refrigeration system, the refrigerant charge in the refrigeration system will usually migrate to the evaporator because the evaporator is usually the coldest spot (lowest pressure) in the refrigeration system. If the refrigeration system is started with a substantial amount of refrigerant in the evaporator, the liquid refrigerant in the evaporator may be pulled into the compressor in sufficient quantities to damage the compressor. That is, undesirable flooding of the refrigeration system compressor with liquid refrigerant from the evaporator may occur at startup of the refrigeration system if large enough quantities of refrigerant collect in the evaporator during an off period of the refrigeration system.
To protect refrigeration system compressors from flooding at startup of the refrigeration system, the refrigeration system may be operated with a pump down or pump out cycle. A pump down cycle comprises pumping the evaporator down to a relatively low pressure at the end of a run period of the refrigeration system to pull substantially all the refrigerant charge out of the evaporator. If the refrigeration system is only off for a short period this works well, but if the off period is relatively long then the pump down cycle may not be effective because refrigerant will gradually migrate back to the evaporator after completion of the pump down cycle. A pump out cycle comprises pumping the evaporator out whenever refrigerant pressure in the evaporator increases to a fixed set point. However, this can result in loss of lubricating oil for the compressor when the compressor is only operated for relatively short run times.